Why this matters:

•       Too much of a specific type of Bt corn — genetically modified to produce insecticides against corn rootworm — is being planted in places that don’t have a high risk of corn rootworms destroying corn crops.
•       This overuse is causing corn rootworms to become resistant, or immune, to Bt insecticides. So Bt corn isn’t working as well now in Corn Belt states where corn rootworm is a serious risk, as rootworms are becoming increasingly pesticide resistant.
•       Corn rootworm is one of the worst pests for corn in the U.S. — it can cost over $1 billion in crop damage per year.

A new study from Michigan State University shows that planting too much genetically modified corn designed to fight off a tough insect — the corn rootworm — especially in the eastern U.S. Corn Belt states may be causing more harm than good.

Bt corn was created to produce natural pesticides, through the introduction of genes from a soil bacterium called Bacillus thuringiensis, or Bt for short. Some varieties of Bt corn kill corn rootworms by making the roots poisonous to them, though the corn is safe for humans and animals. But after years of research across 10 states in the U.S. Corn Belt, scientists have found that the more this corn is planted, the less effective it becomes.

Felicia Wu is a John A. Hannah Distinguished Professor at MSU's College of Agriculture and Natural Resources.

According to study author, Felicia Wu, a John A. Hannah Distinguished Professor at MSU, whose graduated doctoral student Ziwei Ye served as lead author of the study, the U.S. is the largest field corn producing nation in the world, supplying nearly 40% of global corn exports.

“Field corn in the U.S. is used for human food, animal feed and ethanol production. We eat field corn in our cornmeal, grits, corn chips, corn flakes and corn tortillas — not as corn on the cob,” Wu said. “But field corn is plagued by the corn rootworm, which is one of the toughest, most amazing pests that exists. I like to think of the corn rootworm as ‘Loki’ because it’s a trickster — it just keeps evolving resistance to everything we throw at it, not just pesticides or Bt corn, but even crop rotation. And it’s costing farmers billions of dollars.”

So, why has ‘Loki’ become increasingly resistant to the built-in protection of Bt corn? Wu and this research team found farmers were planting more Bt corn than they needed, especially in the eastern Corn Belt states — Ohio, Michigan, Pennsylvania — with no added benefit and higher seed costs. These eastern states don’t have nearly as much of a corn rootworm problem as the major corn producers in the West, which include Iowa, Illinois and Nebraska.

“The problem is that farmers in the east Corn Belt states are overplanting Bt corn even though rootworm is not a problem,” Wu said. “So, we’re putting a lot of the Bt corn with roots containing toxins for corn rootworms into the environment. What this means is that the rootworm is going to evolve resistance more quickly, since it is Loki.

“One of the things that we’ve known in agriculture in general, not just for genetically modified crops, is that the more of a particular pesticide or antibiotic you put in the environment, the more quickly the pests or the bacteria evolve resistance to it because of selection pressure — it’s a matter of survival of the fittest. The organisms will find a way to survive.”

It’s already costing farmers more money because these Bt seeds containing proteins to target corn rootworm are more expensive — and unnecessary in the East. Every time a new trait is introduced to protect a crop, it adds an additional cost per seed. The study estimates that reducing the Bt planting rate from 50% to 18% in eastern states could have saved farmers $99 million in one-year benefits and $1.98 billion in lifetime benefits.

The study calls for more thoughtful and balanced use of Bt corn, including more seed options, including those that don’t protect for corn rootworm, and better communication with farmers. Wu said that to keep this powerful biotech crop working, we need to avoid overusing it and find a new approach that supports both farmers and the environment.

“We need to be more careful about the way we grow Bt corn,” Wu said. “There are less expensive corn varieties that farmers in the East can use that don’t protect against rootworm but do protect against other pests that they have. If we keep using corn rootworm-protected Bt varieties in the East, all we’re doing is shortening the amount of time before the entire industry is negatively impacted.”

The study was published in the journal Science. It was funded in part by the U.S. Department of Agriculture Natural Resources Conservation Service.

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